Differential pressure measuring cells are generally optimized for measuring small pressure differences p1−p2 at high static pressures p1, p2. In doing so, it is important to find the right balance between sensitivity and overload resistance. For example, for the measurement range of the pressure difference |p1−p2|, |p1−p2|/p1<1% may apply. If one of the pressures p1, p2 in a process unit is omitted, the differential pressure sensor is loaded with 100 times the measurement range. Differential pressure sensing elements are known that withstand such overloads. A proven protection of the sensitive differential pressure measuring cells is based upon connecting an overload membrane hydraulically in parallel to the differential pressure sensor and applying the two pressures p1, p2 to the differential pressure measuring cell and the overload membrane via hydraulic paths, wherein the pressures are introduced into the hydraulic paths via separation membranes. An overload membrane has a sufficiently large hydraulic capacity in order to, in case of a unilateral overload, accommodate the volume of a transfer fluid in a hydraulic path to such an extent that the separation membrane of this hydraulic path comes to abut against a membrane bed, so that another increase of the differential pressure acting on the differential pressure sensor is reliably prevented. Examples of differential pressure sensing elements with overload membranes are disclosed in European Patent, EP 1 299 701 B1, and German Patents, DE 10 2006 040 325 A1, and DE 10 2006 057 828 A1.
The use of overload membranes does, however, necessarily lead to larger volume strokes of the transfer fluid and thus, with the same performance capacity, to larger separation membrane surfaces, which results in larger device dimensions and higher costs. In addition, the measuring element dynamics are negatively affected by the overload membrane and the larger volume of the transfer fluid.
Efforts are therefore known for realizing the overload protection for the measuring membrane by means of membrane beds. In doing so, the measuring membrane is to be supported, in case a limit value for a unilateral overpressure is exceeded, by the membrane bed to such a degree that the bursting stress of the measuring membrane is not reached, even in case of another pressure increase.
For this purpose, aspherical membrane beds that approximate the bending line of the measuring membrane at the limit value for the overpressure are especially suitable.
The patent specification U.S. Pat. No. 4,458,537 discloses a capacitive differential pressure measuring cell with an aspherical membrane bed made of glass, which membrane bed is introduced into a structure of coaxial rings, wherein the heights of the rings form a contour that corresponds to the bending line of the measuring membrane.
The published German patent application, DE 10 2009 046 229 A1 discloses a pressure sensor or a differential pressure measuring cell with an aspherical membrane bed made of glass, which membrane bed is formed by thermal sinking.
The patent specification U.S. Pat. No. 7,360,431 B2 discloses a pressure sensor or a differential pressure sensor with an aspherical membrane bed that is prepared in silicon by means of gray scale lithography.
The published German patent application, DE 10 2010 028 773 A1 discloses a pressure sensor or a differential pressure measuring cell with an aspherical membrane bed that is prepared in silicon by means of laser ablation, followed by an oxidation step and a final etching step.
Even though the membrane bed concepts mentioned can, in fact, protect the measuring membrane to a certain degree, the static pressure introduced into the differential pressure measuring cell nonetheless loads the joints between the measuring membrane and the opposing bodies or adjacent regions, so that stress peaks can occur there that result in a destruction of the differential pressure sensor.
The International patent publication, WO 2011/076477 A1 discloses a differential pressure measuring cell, in which the volume stroke of the measuring membrane is sufficient to accommodate the volume of the transfer fluid in case of an overload beneath a separation membrane, without plastic deformation of the measuring membrane.
The still unpublished German patent application, DE 102012113033 discloses a differential pressure sensor with a differential pressure measuring cell that comprises a measuring membrane and opposing bodies made of silicon, wherein the opposing bodies are respectively reinforced on the rear side by a ceramic body in order to avoid or reduce bending of the opposing bodies under static pressures. In this way, the notch stresses on the joints, especially, between the measuring membrane and the opposing bodies are to be reduced.
To the extent that notch stresses occur, especially in cavities having acute angles, approaches are known for avoiding such acute angles between components, which form a chamber, into which a high static pressure is introduced. In this respect, reference is made, for example, to U.S. Pat. No. 5,520,054, which discloses a pressure sensor, the pressure chamber of which has exclusively obtuse angles in its cross-section.
Aside from static overload pressures being present on both sides, a unilateral loading of the differential pressure measuring cell with a static overload pressure can also damage or destroy the measuring membrane, the opposing bodies, or the joints between the measuring membrane and the opposing bodies or adjacent regions, if the unilateral overload results in deformations of the opposing bodies, whereby the supporting function of the membrane beds is impaired, for example.
In order to counteract this, Hein et al. (Transducers '97, pp. 1477-1480, 1997) discloses an encapsulated capacitive differential pressure sensor, in which the opposing bodies are axially clamped between pressure connection pieces, wherein a sealing ring is respectively additionally clamped between an opposing body and a pressure connection piece. The German patent, DE 37 51 546 T2 also discloses a differential pressure sensor that has a measuring membrane between two opposing bodies, wherein the two opposing bodies are axially clamped in an elastic clamping device, in order to increase the bursting strength of the differential pressure sensor. The two arrangements described above have in common that relative movements between the opposing bodies and the clamping device can occur when the differential pressure sensor is loaded with static pressure. This can especially result in hysteresis errors in the zero point and the range of a differential pressure-dependent measurement signal of the differential pressure sensor. The still unpublished German application, DE 102014104831 solves this problem by describing a differential pressure sensor with a clamping device that prevents relative movements between the opposing bodies and the clamping device. These constructions, however, impose high requirements on the component tolerances and are expensive in this respect.
Approaches for hydraulically supporting the differential pressure measuring cell are also known from the prior art. For this purpose, the German published patent application, DE 101 01 180 A1, for example, discloses a differential pressure sensor with an encapsulated differential pressure measuring cell, wherein the differential pressure measuring cell is surrounded in the capsule by a transfer fluid that is kept under pressure by means of a pressure reservoir.
U.S. Pat. Nos. 4,257,274 and 5,684,253 respectively disclose a differential pressure sensor with an isostatically encapsulated differential pressure measuring cell, wherein one of the static pressures that is included in the differential pressure measurement is respectively introduced into a capsule surrounding the differential pressure measuring cell. This concept has a comparatively simple design, but fails when the static overload pressure is the other pressure, i.e., precisely not the pressure that is introduced into the capsule. U.S. Pat. No. 7,624,642 takes this problem into account by the higher of the two process pressures respectively defining the pressure surrounding the differential pressure measuring cell in a capsule, which is achieved via “hydraulic diodes.” However, implementing these is very complex, because additional separation membranes are required for the “hydraulic diodes.”
The above overview of the prior art shows a variety of approaches for making differential pressure sensors suitable for high static pressures, wherein it becomes apparent that none of the solutions mentioned is suitable for all applications, whether for reasons of cost or due to structural or thermomechanical boundary conditions.